1. Field of the Invention
The present invention relates to an image sensor head to be built in an image reading apparatus.
2. Description of the Related Art
Conventionally, an image reading apparatus such as a contact-type scanner incorporates an image capturing unit (image sensor head) utilizing a complementary metal oxide semiconductor (CMOS). Generally, such a CMOS-type image reading apparatus has an advantage that its electric power consumption is lower than that of an image reading apparatus utilizing a charge-coupled device (CCD). However, with respect to the image quality, the CMOS-type apparatus is usually inferior to the CCD-type apparatus. Recently, therefore, an image reading apparatus with an image sensor head which utilizes a CCD sensor as the light receiving means has been put on the market.
Such a prior-art image sensor head has the following structure, for example. The image sensor head includes a rectangular insulating substrate for mounting various components. On the substrate, a plurality of CCD sensor chips are arranged in a row extending longitudinally of the substrate. Each of the sensor chips includes a plurality of light receiving elements arranged in a row extending longitudinally of the substrate. Each sensor chip further incorporates a driver circuit for driving and controlling the light receiving elements. Generally, CMOS elements are used for constituting the driver circuit for the purpose of reducing the electric power consumption. The driver circuit includes a signal amplification section, a switching section and the like.
In the prior-art image sensor head, the plural light receiving elements and the driver circuit for controlling the light receiving elements are built in a single chip. Therefore, in forming the head, the handling of the components is advantageously easy. However, such a “one-chip-structure” causes the following problems.
Generally, the voltage necessary for operating the CCD-type light receiving elements differ from the voltage necessary for operating the driver circuit. Therefore, in the prior-art “one-chip-structure”, after a predetermined drive voltage is supplied from the outside to each of the sensor chips, the supplied voltage need be transformed to a voltage suitable for operating the light receiving elements or to a voltage suitable for operating the driver circuit (or to both) by using a transformer circuit provided in the chip. However, such transforming is not desirable, because it causes power dissipation (an increase in the power consumption).
Moreover, in the prior-art “one-chip-structure”, the following problem occurs in trying to enhance the image reading accuracy. To enhance the image reading accuracy, the number of light receiving elements in the CCD sensor chip need be increased. However, when the number of the light receiving elements is just increased, the time required for reading one line increases. Therefore, to increase the image reading speed, measures need be taken such as using of a high-speed-type amplifier and the like for the driver circuit and increasing of the frequency of a control clock signal. Generally, as the frequency of the control clock signal increases, the load on the driver circuit increases, which leads to an increase in the electric power consumption in the driver circuit. As described above, in the prior art “one-chip-structure”, the light receiving elements and the driver circuit share a single driving power source. Therefore, when the power consumption in the driver circuit increases, a sufficient drive voltage may not be supplied to the light receiving elements. In such a case, the light receiving elements do not operate properly so that the image reading accuracy may be deteriorated.